1. Field of the Invention
The present invention relates to digital image processing and particularly a technique to improve the brightness distribution of a digital image.
2. Description of the Related Art
In order to obtain an image showing suitable brightness and contrast, a method has been known which analyzes a histogram of a captured image and information about an object to execute gradation correction. Particularly, in a backlight scene in which brightness of a main object is significantly dark compared with brightness of a background, since the portion of the main object in a captured image darkens by any means, gradation correction is effective.
The gradation correction includes a method which uses a correction table (tone curve) to convert each input luminance value into a predetermined output luminance value. Particularly with respect to the backlight scene, a method is discussed which sets a correction table that suppresses correction to prevent overexposure of a bright area in the background while the main object is corrected to suitable brightness (refer to Japanese Patent Application Laid-Open No. 2007-124604). Further, a method is discussed which executes gradation correction using a low frequency component signal of a luminance component of an image to obtain effect such as dodging process (refer to Japanese Patent Application Laid-Open No. 2008-072604). In this method, gradation correction can be executed while contrast of an image is maintained compared with tone curve.
However, in Japanese Patent Application Laid-Open No. 2007-124604, gradation correction is executed such that a dark portion of an image is corrected to be bright and correction of an originally bright portion is suppressed by tone curve. Thus, when gradation correction is executed by this method, in a luminance band in the middle of the main object and the background, contrast is lost and a smooth expressionless image is obtained. Further, in Japanese Patent Application Laid-Open No. 2008-072604, when gradation correction is executed using a low frequency component signal of a luminance component of an image, in some scenes, too much contrast appears, so that unnatural image is obtained.